Differential amp drives high-speed converters

The latest high-performance cellular infrastructure, such as 3G and 4G equipment and broadband wireless such as WiMAX, as well as test and medical instrumentation, all require higher speed performance analog-to-digital converters (ADC) than the prior generations of equipment. As a result, the bar has also been raised for the components that drive these A/D converters. Hence, Analog Devices Inc. (ADI) has pushed the envelope for differential amplifiers with its AD8352, which drives up to 14-bit ADCs at speeds of 180MHz. The previous generation of amplifiers drove ADCs up to 70MHz. In their current incarnation, the AD8352 devices can surprisingly even push beyond 180MHz up to 380MHz, with a trade-off in performance.

Converters have improved in speed, resolution, and linearity, and they handle signals with better purity. As these converters push to better performance levels, the amplifiers that drive them need to operate with wider bandwidths, higher resolution, and lower distortion. This part addresses key needs in terms of distortion, linearity, noise, and bandwidth by demonstrating -81dBc/-82dBc typical 2nd/3rd harmonic distortion, 41dBm output third-order intercept point (IP3), 2.6nV/Hz noise spectral density, and a 15.5dB noise figure at 180MHz.

Besides its impressive specifications, what makes the AD8351 special is the ability to set the gain with one external resistor, in a fully differential configuration, which alleviates the need for a transformer. It also offers considerable power savings. For example, as compared to a discrete amplifier design, which draws approximately 100mA to 150mA, this part draws only 37mA with a lower external component count.

So what was the biggest challenge with this design? "The biggest challenge was to get the HD3 (third-order harmonic distortion) specification as low as possible out to the high frequencies," recalls Phil Halford, ADI's marketing manager for RF/IF products. "To achieve this we did two major things, moved to a state-of-the-art SiGe process and used a proprietary distortion cancellation scheme inside the chip, which is a novel architecture that allows the user to adjust the performance for a given operating frequency.

This is a second-generation product for ADI, based on the AD8351 that was released in 2002 with HD3 of -81dBc at 70MHz. "We are specifying the 8352 with HD3 of -82dBc at 180MHz, and I expect the actual performance will be better than that," adds Halford. The team expects to continue to evolve this technology to reach out to 240MHz and 380MHz with even lower distortion.

"This part will enable designers to take full advantage of the latest high-performance ADCs," concludes Halford. The AD8352 is also available as part of the company's ADC evaluation boards.

What is an ADC?ADC stands for analogue-to-digital converter. It is an electronic device that converts input analog voltage to a discrete digital number.Applications include anything that involves the processing of analogue signals into digital form.